Light emitting device

ABSTRACT

A light emitting device has a light emitting element, and a phosphor layer of phosphor glass to generate fluorescence while being excited by light emitted from the light emitting element. The light emitting element emits ultraviolet light, and the phosphor glass generates visible fluorescence while being excited by the ultraviolet light.

[0001] The present application is based on Japanese patent applicationNo. 2003-063016, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a light emitting device and,particularly, to a light emitting device with a phosphor layer forwavelength-converting light emitted from a light emitting element.

[0004] 2. Description of the Related Art

[0005] Conventionally, a light emitting device is suggested that allowsthe wavelength conversion of light emitted from a light emitting elementby a phosphor material to obtain a desired emission color.

[0006] Such a light emitting device is, as shown in FIG. 1, composed of:a board 5; a circuit pattern 4 formed on the board 5; a light emittingelement 1 mounted through the circuit pattern 4 on the board 5, thelight emitting element 1 being enabled to emit ultraviolet light; acover 2 that is formed semispherical while covering a light emittingelement 1 to emit ultraviolet light, and a phosphor layer 3 that isprovided on the inner surface of the cover 2. In the light emittingdevice, the phosphor material in phosphor layer 3 is excited byultraviolet light emitted from the light emitting element 1 and,thereby, it radiates, e.g., white light.

[0007] Japanese patent application laid-open No. 2001-217466 discloses alight emitting device that its light emitting element is covered withsealing resin material (phosphor layer) with phosphor particlesdispersed therein.

[0008] However, in the light emitting device as shown in FIG. 1, sincethe phosphor layer 3 is exposed, the phosphor material may be subjectedto degradation due to absorbed moisture. If an air-tight housing isemployed to prevent the penetration of water into the cover 2, themanufacturing cost will rise since the light emitting device becomesdifficult to assemble. Even in case of dispersing phosphor particles inthe sealing resin material as in Japanese patent application laid-openNo. 2001-217466, the degradation of phosphor material due to moisturecannot be prevented sufficiently.

[0009] Further, since a phosphor material including metal elementgenerally has a high specific density, it may flow by gravitation whenbeing coated on the inner surface of cover 2. Therefore, it is difficultto make the thickness of phosphor layer 3 equal. Thus, it is difficultto generate equal fluorescence over the entire cover 2.

SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a light emittingdevice that the degradation of phosphor material due to moisture can beprevented.

[0011] It is another object of the invention to provide a light emittingdevice that equal fluorescence can be obtained over its entire emissionsurface.

[0012] According to the invention, a light emitting device comprises:

[0013] a light emitting element; and

[0014] a phosphor layer that is composed of phosphor glass to generatefluorescence while being excited by light emitted from the lightemitting element;

[0015] wherein the light emitting element emits ultraviolet light, andthe phosphor glass generates visible fluorescence while being excited bythe ultraviolet light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The preferred embodiments according to the invention will beexplained below referring to the drawings, wherein:

[0017]FIG. 1 is a side view showing the conventional light emittingdevice;

[0018]FIG. 2 is a cross sectional view showing a light emitting element10 used in preferred embodiments of the invention;

[0019]FIG. 3 is a side view showing a light emitting device in a firstembodiment of the invention;

[0020]FIG. 4 is a front view showing the light emitting device in FIG.3;

[0021]FIG. 5 is a side view showing a light emitting device in a secondpreferred embodiment of the invention;

[0022]FIG. 6 is a cross sectional view showing a light emitting devicein a third preferred embodiment of the invention;

[0023]FIG. 7 is an enlarged cross sectional view showing a phosphorlayer 45 and its vicinity in FIG. 6;

[0024]FIG. 8 is an enlarged cross sectional view showing a modificationof phosphor layer 45 in FIG. 6;

[0025]FIG. 9 is a cross sectional view showing part of a light emittingdevice in a fourth preferred embodiment of the invention;

[0026]FIG. 10 is a cross sectional view showing a light emitting devicein a fifth preferred embodiment of the invention;

[0027]FIG. 11 is a cross sectional view showing a light emitting deviceas a modification of the fifth embodiment; and

[0028]FIG. 12 is a cross sectional view showing a light emitting deviceas another modification of the fifth embodiment.

[0029] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] First of all, the components of a light emitting device of theinvention are detailed below.

Light Emitting Element

[0031] The light emitting element includes a light emitting diode, alaser diode or the like. The emission wavelength of light emittingelement is not specifically limited. A III group nitride system compoundsemiconductor element effective for emitting ultraviolet light to greensystem light can be used.

[0032] A light emitting element used for emitting ultraviolet light inthe embodiments is III group nitride system compound semiconductor lightemitting element. Available III group nitride system compoundsemiconductors for the III group nitride system compound semiconductorlight emitting element are represented by a general formula:Al_(X)Ga_(Y)In_(1-X-Y)N (0<X≦1, 0≦Y≦1, 0≦X+Y≦1). Those composed of Alinclude two-element system such as AlN, and three-element system such asAl_(X1)Ga_(1-X1)N and Al_(X1)In_(1-X1)N (0<x1<1). In the III groupnitride system compound semiconductors and GaN, at least part of IIIgroup element may be replaced by boron (B), thallium (Tl) etc. and atleast part of nitrogen (N) may be replaced by phosphorous (P), arsenic(As), antimony (Sb), bismuth (Bi) etc.

[0033] The III group nitride system compound semiconductor may includean arbitrary dopant (impurity) n-type impurity available is silicon(Si), germanium (Ge), selenium (Se), tellurium (Te), carbon (C) etc.p-type impurity available is magnesium (Mg), zinc (Zn), beryllium (Be),calcium (Ca), strontium (Sr), barium (Ba) etc. After doping p-typeimpurity, the III group nitride system compound semiconductor can betreated by a known method such as electron beam radiation. annealing andplasma radiation to lower the resistivity, but this treatment is notnecessarily needed.

[0034] III group nitride system compound semiconductor layer can beformed by MOCVD (metal organic chemical vapor deposition). It is notalways necessary to form all semiconductor layers to compose the lightemitting element by MOCVD. Instead of or together with MOCVD, molecularbeam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), sputtering, ionplating, electron shower etc. may be used.

[0035] The light emitting element may include MIS junction, PIN junctionor pn-junction included homo-structure, hetero-structure or doublehetero-structure. Light emitting layer thereof may include quantum wellstructure (single quantum well structure or multi-quantum wellstructure). The III group nitride system compound semiconductor lightemitting element may be of a face-up type that main light emissiondirection (electrode surface) is in the optical axis direction of lightemitting device or a flip-chip type that main light emission directionis in the opposite direction to the optical axis direction to usereflected light.

Phosphor Glass

[0036] Phosphor glass includes a fluorescence activation element as oneof glass components. In other words, it is composed such that an elementhaving fluorescence activity, such as a rare-earth element, is dopedinto a basic glass component such as fluorophosphate.

[0037] For example, fluorophosphate phosphor glass, which is disclosedin Japanese patent application laid-open Nos. 8-133780 and 9-202642, andoxide phosphor glass, which is disclosed in Japanese patent applicationlaid-open No. 10-167755 can be used as phosphor glass of the invention.

[0038] The phosphor glass is doped with a fluorescence activationelement such as Tb³⁺, Eu²⁺ and Eu³⁺. Although the phosphor glass issubstantially colorless and transparent under visible light, itgenerates intensive fluorescence to ultraviolet light in a widewavelength range. Being irradiated with ultraviolet light. Tb³⁺generates green system fluorescence, Eu²⁺ generates blue systemfluorescence, and Eu³⁺ generates red system fluorescence. Such phosphorglass doped with the fluorescence activation element is commerciallyavailable as Lumilass G9, Lumilass B and Lumilass R7 (each registeredtrade mark) which are manufactured by SUMITA Optical Glass, Inc., one ofthe assignees.

Phosphor Layer

[0039] A phosphor layer functions such that it generates fluorescence bybeing irradiated with light from a light emitting element whilewavelength-converting that light to define an emission color of thelight emitting device. The phosphor layer is not specifically limited inshape and disposition etc. as far as it can receive light from the lightemitting element and discharge fluorescence out of the light emittingdevice, and it may be suitably designed according to the characteristicsof light emitting device.

[0040] The phosphor glass described earlier may, by itself, compose thephosphor layer. The fluorescence activation element is evenly contained,as one glass component, in the phosphor glass. Therefore, when thephosphor layer is of phosphor glass, there occurs no unevenness offluorescence in the phosphor layer. Also, since the fluorescenceactivation element is incorporated into the glass structure, it can bestabilized to moisture.

[0041] The phosphor layer may be composed stacking different kinds ofphosphor glass layers. By mixing fluorescence's generated from thephosphor glass layers, the emission color of light emitting device canbe arbitrarily controlled. For example, when stacked phosphor glasses ofTb³⁺ (green system fluorescence), Eu³⁺ (blue system fluorescence) andEu³⁺ (red system fluorescence) are applied to an ultraviolet lightemitting element, fluorescence's generated from the phosphor glasses aremixed to produce white light.

[0042] The phosphor glass described earlier may be contained (dispersed)in the phosphor layer while being ground into particles or power. Evenif it is used dispersed in resin, the fluorescence characteristic can bemaintained for a long period due to its excellent water resistance, ascompared to a general phosphor material.

[0043] The grinding of phosphor glass can be conducted using a knowngrinding apparatus such as a ball mill.

[0044] Particles of phosphor glass obtained by the grinding aredispersed in a transparent (light-transmitting) material to form thematrix of phosphor layer. Several kinds of phosphor glass particles maybe dispersed in the transparent material. Thereby, the emission color oflight emitting device can be arbitrarily controlled.

[0045] Herein, a particle of phosphor glass means a particle to beobtained by grinding a bulk of phosphor glass, and its shape may be invarious forms such as particle, power and flake.

[0046] A known phosphor material may be dispersed in the transparentmaterial. For example, when using alight emitting element to emitultraviolet light, available with the particle of phosphor glass is anyone or a combination of two or more selected from phosphors: ZnS:Cu, Al;(Zn, Cd)S:Cu, Al; ZnS:Cu, Au, Al; Y₂SiO₅:Tb; (Zn, Cd)S:Cu; Gd₂O₂S:Tb;Y₂O₂S:Tb; Y₃Al₅O₁₂:Ce; (Zn, Cd)S:Ag; ZnS:Ag, Cu, Ga, Cl; Y₃Al₅O₁₂:Tb;Y₃(Al, Ga)₅O₁₂:Tb; Zn₂SiO₄:Mn; LaPO₄:Ce, Tb; Y₂O₃S:Eu; YVO₄:Eu; ZnS:Mn;Y₂O₃:Eu; ZnS:Ag, ZnS:Ag, Al; (Sr, Ca, Ba, Mg)₁₀(PO₄)₆Cl₂:Eu;Sr₁₀(PO₄)₆Cl₂:Eu; (Ba, Sr, Eu) (Mg, Mn)Al₁₀O₁₇; (Ba, Eu)MgAl₁₀O₁₇;ZnO:Zn; and Y₂SiO₅:Ce.

[0047] A second phosphor material may be further combined that can beexcited by fluorescence light (a) generated from the phosphor glassand/or phosphor material and can generate fluorescence light (b) with awavelength different from the fluorescence light (a).

[0048] The particle of phosphor glass and/or phosphor material may beevenly dispersed or locally disposed in the transparent material. Forexample, when the particles of phosphor glass are locally disposed nearthe light emitting element, light to be emitted from the light emittingelement can be effectively wavelength-converted by them.

[0049] A light diffusion material can be contained (dispersed) in thetransparent material so as to diffuse light in the transparent material.Thereby, unevenness in emission color can be reduced.

[0050] The transparent material may be epoxy resin, silicon resin, urearesin, glass etc. Especially, low-melting glass of SiO₂—Nb₂O₅ system,B₂O₃—F system, P₂O₅—F system, P₂O₅—ZnO system, SiO₂—B₂O₃—La₂O₃ system orSiO₂—B₂O₃ system is preferable. These low-melting glasses do not containany harmful element. Thereby, a good work environment in manufacturingprocess can be easily secured and a load in recycling can be reduced.Since the low-melting glass is chemically stable as compared to resinmaterial, it is not subjected to discoloration such as yellowing. Sincetemperature in covering the light emitting element with the low-meltingglass is controlled to be relatively low, the light emitting element orthe other components such as bonding wire is not affected by heat.Further, because of having no resin component, the entire device cansecure a high heat resistance. Therefore, the light emitting device canbe applied to a heat processing such as solder reflowing.

[0051] Of these low-melting glasses, one or two or more to bearbitrarily selected may be used.

[0052] According to intended use and use conditions, the concentrationdistribution of phosphor glass particles in the transparent material maybe changed. Namely, the amount of phosphor glass particles may be variedsequentially or stepwise with being close to the light emitting element.For example, the concentration of phosphor glass particles can beincreased near the light emitting element. Thereby, light from the lightemitting element can be irradiated to phosphor glass particles. Ifconcentration of phosphor glass particles is reduced with being close tothe light emitting element, the degradation of phosphor glass particlesdue to heat from the light emitting element can be prevented.

[0053] Between phosphor material and light emitting element, a layer ofanother transparent material or a space may be provided.

FIRST EMBODIMENT

[0054]FIG. 2 is a cross sectional view showing a light emitting element10 used in the preferred embodiments of the invention.

[0055] In this embodiment, a face-up type III group nitride systemcompound semiconductor light emitting element 10 as shown in FIG. 2 isused. The light emitting element 10 emits ultraviolet light. The detailsof layers to compose the light emitting element 10 are as follows:Layers Composition p-type layer 15 p-GaN:Mg layer 14 including InGaNlayer included light emitting layer n-type layer 13 n-GaN:Si bufferlayer 12 AlN substrate 11 sapphire

[0056] The emission wavelength of light emitting element 10 can beadjusted by controlling the composition ratio of III group element inlayer 14 including light emitting layer. Alternatively, a flip-chip typelight emitting element may be used that a thick p-electrode to cover thesurface of p-type layer 15 is provided instead of a transparentelectrode 16 and p-electrode 17.

[0057] n-type layer 13 of GaN with n-type impurity Si doped is grownthrough buffer layer 12 on the substrate 11. Although the substrate 11is of sapphire in this embodiment, it may be, instead, of spinel,silicon carbide, zinc oxide, magnesium oxide, manganese oxide, zirconiumboride or III group nitride system compound semiconductor single crystaletc. The buffer layer 12 of AlN is grown by MOCVD. It may be of GaN,InN, AlGaN, InGaN, AlGaInN etc. and may be grown by molecular beamepitaxy (MBE), hydride vapor phase epitaxy (HVPE), sputtering, ionplating etc. when the substrate 11 is of III group nitride systemcompound semiconductor, the buffer layer can be omitted.

[0058] Meanwhile, the substrate and buffer layer can be removed afterforming the semiconductor element, when needed.

[0059] Although n-type layer 13 is of GaN, it may be of AlGaN, InGaN orAlInGaN.

[0060] Although n-type impurity Si is doped in the n-type layer 13, theother n-type impurity, e.g. Ge, Se, Te and C may be doped therein.

[0061] The layer 14 including light emitting layer can have quantum wellstructure (multiquantum well structure or single quantum wellstructure). The structure of light emitting element maybe ofsingle-hetero type, double-hetero type or homo junction type.

[0062] The layer 14 including light emitting layer may also includeMg-doped III group nitride system compound semiconductor layer with alarge bandgap on the p-type layer 15 side. This prevents electronsinjected into the layer 14 including light emitting layer fromdispersing into the p-type layer 15.

[0063] p-type layer 15 of GaN with p-type impurity Mg doped is grown onthe layer 14 including light emitting layer. The p-type layer IS may beof AlGaN, InGaN or AlInGaN. p-type impurity may be Zn, Be, Ca, Sr or Ba.After doping p-type impurity, the layer 15 can be treated by a knownmethod such as electron beam radiation, annealing and plasma radiationto lower the resistivity, but this treatment is not needed necessarily.

[0064] In this embodiment, the III group nitride system compoundsemiconductor layer is grown, under general conditions, by MOCVD,molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE),sputtering, ion plating etc.

[0065] n-type electrode 18 is composed of two layers of Al and V. Aftergrowing the p-type layer 15, part of the p-type layer 15, layer 14including light emitting layer and n-type layer 13 is removed byetching, then forming the n-type electrode 18 on exposed n-type layer 13by vapor deposition.

[0066] Transparent electrode 16 formed on the p-type layer 15 is thinfilm including gold. Also, p-type electrode 17 formed on the transparentelectrode 17 by vapor deposition is of material including gold.

[0067] After forming the layers and electrodes as described above, thewater is separated into chips.

[0068]FIG. 3 is a side view showing a light emitting device 20 in thefirst embodiment of the invention FIG. 4 is a front view showing thelight emitting device in FIG. 3.

[0069] The light emitting elements 10 are disposed at equal intervals ona board 22 with a circuit pattern 21 formed thereon. Asemicylinder-shaped cover 23 (phosphor layer) is bonded onto the board22 while covering the light emitting element 10. The cover 23 is ofphosphor glass and its both ends are opened. In this embodiment, thephosphor glass is fluorophosphate glass with Tb³⁺ doped therein(Lumilass G9 (registered trade mark): SUMITA Optical Glass, Inc.).

[0070] In the light emitting device 20, ultraviolet light emitted fromthe light emitting element 10 excites phosphor element (Tb³⁺) in thecover 23 to generate green system fluorescence. The phosphor element isevenly doped in phosphor glass to compose the cover 23. Thereby, thefluorescence can be generated evenly and, therefore, unevenness inemission color can be prevented. The phosphor element as part of glassmaterial is not subjected to degradation due to moisture. Thus, thelight emitting device has an excellent endurance.

[0071] In this embodiment, by varying the amount of phosphor element atthe stage of preparing the phosphor glass, the color strength offluorescence can be controlled. In contrast, with the conventional lightemitting device as shown in FIG. 1, the color strength of fluorescencecan be controlled by varying the thickness of phosphor material layer.However, it is difficult to finely control the thickness and, therefore,unevenness in thickness may occur.

SECOND EMBODIMENT

[0072]FIG. 5 is a side view showing a light emitting device 30 in thesecond preferred embodiment of the invention. Like components areindicated by the same numerals used in FIG. 3.

[0073] As shown in FIG. 5, the light emitting device 30 is provided witha cover 33 composed of three layers (phosphor layer). A first layer 33Ron the outer surface is of fluorophosphate glass with Eu³⁺ doped therein(Lumilass R7 (registered trade mark): SUMITA Optical Glass, Inc.), asecond layer 33G at the center is of fluorophosphate glass with Tb³⁺doped therein (Lumilass G9(registered trade mark): SUMITA Optical Glass,Inc.), and a third layer 33B on the inner surface is of fluorophosphateglass with Eu²⁺ doped therein (Lumilass B(registered trade mark): SUMITAOptical Glass, Inc.). The layers 33R, 33G and 33B are to generatered-system fluorescence, green-system fluorescence and blue-systemfluorescence, respectively. By mixing these three-color fluorescencelights, white-system light can be radiated from the cover 33.

[0074] In the first and second embodiments, the shape of cover may bedesigned arbitrarily. When a rare-earth element as phosphor element isdoped in the low-melting glass, the cover can be formed by molding.Thereby, a degree of freedom in designing can be further enhanced.

THIRD EMBODIMENT

[0075]FIG. 6 is a cross sectional view showing a light emitting device40 in the third preferred embodiment of the invention.

[0076] In the light emitting device 40, the light emitting element 10 isdisposed at the bottom of a cup portion 42 of a mount lead 41. The cupportion 42 is filled with a phosphor layer 45, and the mount lead 41 andsub-lead 43 are sealed with a lamp-shaped seal member 47.

[0077]FIG. 7 is an enlarged cross sectional view showing the phosphorlayer 45 and its vicinity in FIG. 6.

[0078] The phosphor layer 45 is composed of a transparent material 46 ofepoxy resin etc. and ground phosphor glass particles 48, which are offluorophosphate glass with Tb³⁺ doped therein (Lumilass G9(registeredtrade mark): SUMITA Optical Glass, Inc.), being dispersed in thetransparent material 46. Since the phosphor glass has a specific densitythat is not significantly greater than that of a general phosphormaterial such as YAG system phosphor, it can be evenly dispersed in thetransparent material. The phosphor glass is not subjected to degradationdue to moisture. Thus, the light emitting device has an excellentendurance.

[0079] Although in this embodiment the same kind of phosphor glassparticles 48 are dispersed in the transparent material 46, phosphorglass particles with different emission colors may be dispersed therein.In such a case, although general phosphor materials with differentemission colors are different one another in specific density, if thephosphor glass particles are composed of the same glass material so asto have the same specific density, they can be further equally mixed.Thereby, unevenness in emission color can be prevented.

[0080]FIG. 8 is an enlarged cross sectional view showing a modificationof phosphor layer 45 in FIG. 6. As shown, a phosphor material 49 isdispersed in a transparent material 47 as well as phosphor glassparticles 48. In this modification, the transparent material 47 ofhigh-viscosity silicone resin is used to prevent the precipitation ofphosphor material 49 with a high specific density.

[0081] On the other hand, a phosphor glass power to generate redfluorescence may be added to a phosphor material short of redfluorescence element. Thereby, the color balance can be adjusted. Also,the light absorption in phosphor layer can be suppressed low since lightexcept for light to be absorbed by the phosphor element is transmittedthrough the phosphor glass whereas the red phosphor material does nothave a high emission efficiency and causes reflection and diffusion oflight. Further, the color tone can be effectively stabilized since thered phosphor material does not have a high emission efficiency and islikely to be precipitated due to its specific density (about 6 to 7)higher than the other phosphor materials (about 3.0 for green phosphormaterials and about 6.2 for blue phosphor materials) thereby causing acolor separation.

FOURTH EMBODIMENT

[0082]FIG. 9 is a cross sectional view showing part of a light emittingdevice 50 in the fourth preferred embodiment of the invention.

[0083] In the light emitting device 50, the light emitting element 10 ismounted on the end of a mount lead 51. In FIG. 9, 52 is a sub-lead, and56, 55 are bonding wires. In this embodiment, a phosphor layer 57 iscomposed of a matrix of low-melting glass 53 and ground phosphor glassparticles 58, which are composed of fluorophosphate glass with Eu³⁺doped therein (Lumilass R7(registered trade mark): SUMITA Optical Glass,Inc.), fluorophosphate glass with Tb³⁺ doped therein (LumilassG9(registered trade mark); SUMITA Optical Glass, Inc.), andfluorophosphate glass with Eu²⁺ doped therein (Lumilass B (registeredtrade mark): SUMITA Optical Glass, Inc.) being dispersed in thelow-melting glass 53. The phosphor layer 57 is formed semicircular bypress-molding the low-melting glass 53.

[0084] In the light emitting device 50, ultraviolet light emitted fromthe light emitting element 10 excites the ground phosphor glassparticles 58, each of which generates red-system fluorescence,green-system fluorescence or blue-system fluorescence. By mixing thesethree-color fluorescence lights, white-system light can be radiated fromthe entire light emitting device 50.

FIFTH EMBODIMENT

[0085]FIG. 10 is a cross sectional view showing a light emitting device60 in the fifth preferred embodiment of the invention.

[0086] The light emitting device 60 is composed of a light emittingelement 10, a metallic stem 63 to mount the light emitting element 10thereon, a housing 61 and a window 62 as phosphor layer. The window 62is of fluorophosphate glass with Tb³⁺ doped thinly therein.

[0087] In the light emitting device 60, when ultraviolet light emittedfrom the light emitting element 10 is transmitted through the window 62,phosphor glass to compose the window 62 generates thinly greenfluorescence.

[0088] Since phosphor element (Tb³⁺) in the phosphor glass radiatesfluorescence to all directions, that fluorescence can be checked by eyesin the direction other than the optical axis. The phosphor glass hassuch a characteristic that light, of light emitted from the lightemitting element 10, not to be absorbed by the phosphor element isdirectly passed through. Therefore, the emission of light emittingelement 10 can be easily checked by eyes from the side of light emittingdevice 60. Ultraviolet light itself can be radiated outside withoutbeing affected in its light output and degree of convergence.

[0089] FIG.11 is a cross sectional view showing a light emitting device70 as a modification of the fifth embodiment.

[0090] The light emitting device 70 is provided with a convexlens-shaped window 64. Like components are indicated by the samenumerals used in FIG. 10.

[0091] In the light emitting device 70, ultraviolet light emitted fromthe light emitting element 10 is converged by the window 64. Therefore,intensive ultraviolet light can be radiated in the optical axisdirection. The emission spectrum of light emitting element 10 includes abit of visible light region, though it mostly belongs to ultravioletlight region. Therefore, the turn-on state of light emitting element 10can be checked by eyes and, however, it is undesirable to check theturn-on state from a convergence direction along the optical axis. Inthis embodiment, where the window 64 is composed of phosphor glass, theturn-on state of light emitting element 10 can be checked by observingfluorescence light to be radiated laterally from the window 64 whilesecuring the safety.

[0092]FIG. 12 is a cross sectional view showing a light emitting device80 as another modification of the fifth embodiment. 81 is a reflectionmirror. Like components are indicated by the same numerals used in FIG.10.

[0093] Also in the light emitting device 80, the turn-on state of lightemitting element 10 can be checked by observing fluorescence light to beradiated laterally from the window 62 while securing the safety sinceultraviolet light emitted from the light emitting element 10 can beconverged.

[0094] Although the light emitting device of this embodiment is providedwith the window of phosphor glass, it is not necessarily provided withthe window. Alternatively, it may be provided with a member to whichlight from the light emitting element 10 is irradiated and which isplaced at a position where excited light of phosphor glass can bechecked from outward.

[0095] Although the invention has been described with respect to thespecific embodiments for complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A light emitting device, comprising: a light emitting element; and a phosphor layer that is composed of phosphor glass to generate fluorescence while being excited by light emitted from the light emitting element; wherein the light emitting element emits ultraviolet light, and the phosphor glass generates visible fluorescence while being excited by the ultraviolet light.
 2. The light emitting device according to claim 1, wherein: the phosphor glass contains, as glass component, at least one of Tb³⁻ (terbium), Eu²⁺ (divalent europium) and Eu³⁻ (trivalent europium).
 3. The light emitting device according to claim 1, wherein: the phosphor layer is composed of a plurality of layers that are of different kinds of the phosphor glasses.
 4. The light emitting device according to claim 1, wherein: the phosphor glass is particle-shaped, and the phosphor layer is composed of a transparent material and the particle-shaped phosphor glass that is dispersed in the transparent material.
 5. The light emitting device according to claim 4, wherein: the particle-shaped phosphor glass is composed of different kinds of the particle-shaped phosphor glasses.
 6. The light emitting device according to claim 4, wherein: the phosphor layer is further composed of a phosphor material other than the phosphor glass, the phosphor material being dispersed in the transparent material.
 7. The light emitting device according to claim 4, wherein: the transparent material is low-melting glass or synthetic resin.
 8. A light emitting device, comprising: a light emitting element; an optical system that converges light emitted from the light emitting element; wherein the optical system is composed of phosphor glass. 